This invention relates to a vibration actuator using an electro-mechanical transducer including a magnetic circuit and a driving coil and having a damper elastically supporting the magnetic circuit, and in particular to a structure of the damper.
An electro-dynamic type of the electromechanical transducer comprises a magnetic circuit comprising a magnet and magnetic yoke and having a magnetic gap therein, and a moving coil or ribbon disposed in the magnetic gap. When a driving AC current is applied to the moving coil or ribbon, the moving coil or ribbon vibrates relatively to the magnetic circuit. A frequency of the vibration is dependent on a frequency of the driving AC current. Since the moving coil or ribbon is applied with the driving AC current and moves or vibrates, it is referred to as a driving coil and also a moving element.
When the driving AC current is of an audio frequency, the moving coil or ribbon vibrates at the audio frequency. When a thin plate or diaphragm is connected to the moving coil or ribbon directly or through the damper, it is vibrated at the audio frequency to produce sound. This is well known as an electro-dynamic speaker.
On the other hand, an electromagnetic type of the electro-mechanical transducer comprises a magnetic circuit comprising a magnet, magnetic yoke and a driving coil wound on the magnetic yoke and having a magnetic gap formed therein, and a magnetic armature or a small magnetic piece as a moving element disposed in the magnetic gap. When the driving AC current is applied to the driving coil, the magnetic armature vibrates at a frequency of the driving AC current. The electromagnetic type transducer is also used for a speaker where the magnetic armature is connected to a diaphragm or a thin plate.
In the electromechanical transducer of either one of the two types described above, the magnetic circuit can be vibrated at a low frequency which is lower than the audio frequency by supporting the magnetic circuit through a damper onto a rigid support member or frame, by fixing the moving element to the support member directly or through a low compliant elastic member, and by applying to the driving coil a driving AC current of the low frequency. The vibration is transmitted to the support member through the damper. Therefore, when a person attaches the support member or a material fixed to the support, he can feel the vibration through his skin. Thus, the transducer can be used in a vibration actuator for producing a low frequency vibration which a human body can feel through a skin.
In such a vibration actuator, when a driving AC current of the audio frequency is applied to the driving coil, the moving element vibrates at the audio frequency. The vibration is transmitted to the support member. When a thin plate or a diaphragm is joined to the support member, it vibrates to produce an audible sound. Using this principle, a small-size vibration actuator is proposed for producing a voice and a ringing tone, as well as signaling vibration for announcement of call reception in mobile communication (for example, see Japanese Unexamined Patent Applications (JP-A) No. H10-165892 and No. H11-027921.
These Japanese publications disclose a damper having spiral a spring portions for supporting the magnetic circuit as shown in FIG. 5 of JP-A""892 and also in FIG. 5. of JP-A""921. The damper is made of an elastic disk such as a metal plate and comprises an inner ring portion, outer ring portion and a plurality of spiral spring portions connecting between the inner and outer ring portions. The inner ring and the outer ring are fixed to the magnetic circuit and the support frame, respectively.
Each of the spiral spring portions extends from the inner ring portion to the outer ring portion in spiral shape and is defined by an inner spiral slit and an outer spiral slit In the structure, even if the damper is limited in its radius, each of the spiral spring portions has a long size comparing radial spring arms formed within the limited radius. Therefore, the magnetic circuit can be elastically supported by the spring portions with a high compliance comparing with the limited radius of the damper.
In an existing one of the damper having the spiral spring portions, an effective spring length of the spiral spring portion is mainly determined by an angle around a center of the damper from an inner end of the inner spiral slit to an outer end of the outer spiral slit. The angle is hereinafter referred to as xe2x80x9ceffective anglexe2x80x9d. It has been considered to be sufficient to elastically support the magnetic circuit with a relatively high compliance that the effective angle is 55 angular degree at the maximum. The effective angle has been usually selected to be an angle smaller than 55 angular degrees, considering that use of a large effective angle makes it difficult to produce the damper.
However, the above-mentioned existing vibration actuator is disadvantageous in that the damper may often suffer a permanent strain if an abnormal stress is applied by external shock or the like.
After studying the reason of the problem caused, the inventor knew that the existing damper having spiral spring portions with the effective angle smaller than 55 angular degrees cannot provide a sufficient high compliance against any relatively large external force caused due to mechanical shock such as dropping but still exhibits a relatively large stiffness in the radial direction. If subjected to such a large external stress, for example, when the vibration actuator is dropped, the magnetic circuit may abnormally be displaced in the radial direction. Such abnormal displacement may leave the permanent strain in the damper and may further cause the inclination of the center shaft of the magnetic circuit. In case where the strain or the inclination is great, the abnormal stress is applied to the damper so that the stability in characteristics would be deteriorated.
It is therefore an object of the present invention to provide a vibration actuator which is capable of improving a shock resistance to keep stable characteristics and high reliability over a long period of time.
This invention is applicable to a vibration actuator having an electro-mechanical transducer including a driving coil and a magnetic circuit comprising a magnet and yoke. The vibration actuator comprises a support frame and a damper supporting the magnetic circuit onto the support frame. The damper comprises an inner ring portion, an outer ring portion, and a plurality of spiral spring portions connecting the inner and outer rings. Each of the spiral spring portions extends in a spiral shape from the inner ring portion to the outer ring portion and is defined by an inner spiral slit and an outer spiral slit. The damper is characterized in that the effective angle is selected to be an angle larger than 55 angular degrees.
This invention is applicable to a vibration actuator having an electro-mechanical transducer including a driving coil and a magnetic circuit comprising a magnet and yoke. The vibration actuator comprises a support frame and a damper supporting the magnetic circuit onto the support frame. The damper comprises an inner ring portion, an outer ring portion, and a plurality of spiral spring portions connecting the inner and outer rings. Each of the spiral spring portions extends in a spiral shape from the inner ring portion to the outer ring portion and is defined by an inner spiral slit and an outer spiral slit. Each of the spiral spring portions has an effective spring length of 320 or more, preferably, 400 or more. The effective spring length is determined by a product (rxc2x7xcex8) of an average radius (r) and an effective angle (xcex8) of the spiral spring portion.
The effective angle is determined as an angle (by angular degree) from an inner end of the inner spiral slit to an outer end of the outer spiral slit defining each respective spiral spring portion around a center of the damper.
The average radius (r) is determined by an average of various distances from the damper center to various points on a spiral curve extending along a central line between the inner and outer spiral slits from an inner end to an outer end of the spiral spring portions, that is, from a home angular position of the effective angle to a terminal angular position moved by an angle of the effective angle xcex8.
The average radius is approximately given by an average ((D0+Dxcex8)/2) of one (D0) of the various distances at the home angular position of the effective angle and another (Dxcex8) at the terminal angular position.
Alternatively, the average radius is approximately given by one (Dm) of the various distances at an angular position moved by an angle of xcex8/2 from the home angular position to the terminal angular position, that is, a distance from the damper center to a midpoint on the spiral curve between the home angular position and the terminal angular position.
With the above-mentioned structure, the effective spring length of the spiral spring portion can be increased so that the stiffness of the damper for the radial shock is reduced. As a result, even if the external stress is applied in the radial direction, for example, when the vibration actuator is dropped, the magnetic circuit is only temporarily displaced in the radial direction and is free from any permanent strain.
Preferably, the damper is formed by at least one non-magnetic metal plate selected from SUS304, SUS301, nickel silver, phosphor bronze, and a Be-Cu alloy or an elastic plastic resin. Preferably, the slits determining the spiral spring portions are formed in a disk of the metal plate and are arranged at a predetermined interval from one another.